The synthesis of cryptate compounds incorporating nitrogen or sulphur atoms was reported by Sargeson et al in the late 70s to early 80s. As shown in Scheme 1, these compounds can be prepared via a simple metal template process, capitalising on the inertness of Co(III) metal complexes. Reduction of the Co(III) complex of 1,8 dinitro-3,6,10,13,16,19-hexaazabicyclo[6.6.6]icosane (Co-“dinosar”, compound 2) to produce the Co(II) complex of [3,6,10,13,16,19-hexaazabicyclo[6.6.6]eicosane-1,8-diamine] (“Co-diamsar”) was afforded with excess zinc metal powder in strong acid. The resultant Co-diamsar was then converted to its Co(III) metal complex (compound 3) with hydrogen peroxide prior to removal of the Co(III) to yield the free ligand (“diamsar”). Co(III) can then be removed from the cryptate compound using high concentrations of hydrochloric or hydrobromic acid (at 130 to 150° C.) or using excess cyanide ion, the latter producing the highest yield.

The Cu(II) complex of diamsar can be used to form other cryptate or cryptand compounds such as 1-N-(4-aminobenzyl)-3,6,10,13,16,19-hexaazabicyclo[6.6.6]eicosane-1,8-diamine (“sarar”, compound 8) via the reactions shown in Scheme 2. In this reaction, the Cu(II) acts as a templating metal tying up the secondary amines in a Cu(II) complex, thus reducing the potential for multiple by-products having substituted secondary amines.

Coupling the Cu(II) complex of diamsar with nitrobenzyl aldehyde via Schiff base condensation in ethanol affords the synthesis of the desired mono-substituted diamsar in reasonable yield (25-30%), which then needs to be separated from the bis-substituted Cu(II) diamsar complex and unreacted Cu(II) diamsar. The separation can be achieved using ion exchange chromatography, but can be challenging. For example, yields are often compromised because of the need to change counter ions and because various forms of the complexes can precipitate on the column. Furthermore, such separations often require many litres of strong acid eluents, which then need to be disposed of. Removal of the Cu(II) is achieved under reducing conditions, for example, using palladium/charcoal and sodium borohydride, which also causes the nitrate to be converted into an aromatic amine. Minor variations of this reaction scheme have been used to make compounds in which diamsar is coupled to molecules other than nitrobenzyl aldehyde.
Sarar can be conjugated to a range of carrier agents, such as molecular recognition units, and used for imaging and therapy. For example, sarar-immunoconjugates have been demonstrated to be useful for 64Cu PET imaging and radiotherapy. Sarar is able to selectively complex 64Cu2+ rapidly (within minutes) over a wide range of pH (4-9) values. The 64Cu PET radiolabelling can take place at room temperature and results in kinetically inert complexes that are stable to excess EDTA challenge. Thus, high specific activity radiopharmaceutical products are easily prepared without the need for specialised skills or infrastructure and without requiring further purification steps.
It would be advantageous to provide alternative methods by which cryptand and/or cryptate compounds can be coupled with other molecules.